This invention relates to output buffers, and more particularly to reducing undershoot and ground bounce for high-drive CMOS output buffers.
Much development effort has gone into reducing noise created by output buffers that drive signals off of an integrated circuit (IC) substrate. As transistor device sizes, especially gate length, are reduced, larger currents can be switched more quickly.
While fast switching of large currents is useful, it has an unwanted side effect. Sudden changes in current can produce ground-voltage bounce as inductances in IC packages resists changes in current. Such changes in the ground voltage inside an IC package can lead to false triggering of other inputs, or other noise problems.
Despite such ground-bounce problems, high current drivers are still useful for some IC applications. IC output buffers or drivers that drive external buses or signal lines may need to drive a large current to compensate for currents through terminating and other resistances external to the IC chip.
The static current drive of an output buffer is often indirectly specified by a voltage-output-low (VOL) and voltage-output-high (VOH) requirement. Under the VOL test conditions, the output buffer must maintain the output voltage below the specified VOL when a specified amount of current is sunk through the output buffer from an external source, such as a tester machine.
To meet the VOL requirement, the effective resistance of a pull-down transistor in the output buffer can be reduced, thus reducing the voltage drop through the output buffer. The effective resistance can be reduced by using a larger, higher-current-driver transistor as the pulldown. Using such a large transistor has the disadvantage that noise is often increased. When the output buffer switches state, the larger transistor produces a larger switching current, resulting in increased inductive ground bounce. The output voltage can then undershoot ground, producing ringing and oscillation of the output voltage.
Many noise-reduction techniques have been employed. The pull-down transistor can be split into several smaller pull-down transistors that are turned on in a sequence, rather than all at once. The slew rate of the signal activating the pull-down transistor can be carefully controlled or shaped. Feedback techniques can also be used.
While these techniques have been somewhat successful at reducing ground bounce, a high output current to meet the static VOL test is still desirable.